<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>p2_外间距</title>
    <style>
        /*去掉body自带的外间距*/
        body{
            margin: 0;
        }
        .fu {
            background-color: rgba(255,255,0,0.3);
            /* 2.父子粘连(贴边):
               首/尾元素的上/下外间距会超出父元素的范围,需要给父元素设置BFC保护*/
            overflow: hidden;/* BFC结界 */
        }
        .fu>div {
            width: 100px;
            height: 100px;
            background: rgba(0,0,255,.6);
            color: #fff;
            border: 2px solid #00f;
            font-size: 25px;
            font-weight: bold;
        }
        .z1 {
            margin-bottom: 30px;
            margin-top: 50px;
        }
        .z2 {
            margin-top: 50px;
        }
        .z3 {
            margin: 10px;
            margin: 10px 20px;
            margin: 10px 20px 30px;
            margin: 10px 20px 30px 40px;
            margin: 0 auto;/*auto会自动计算左右外间距成为相等的值*/
        }
        .z4 {
            margin-bottom: 50px;
        }
        span {
            border: 2px solid #f00;
        }
        .s2{
            /* 行内元素垂直方向的外间距不生效 */
            margin: 30px;
        }
    </style>
</head>
<body>
    <span>span1</span><span class="s2">span2</span><span>span3</span>
<div class="fu">
    <div class="z1">子元素1</div>
    <div class="z2">子元素2</div>
    <div class="z3">子元素3</div>
    <div class="z4">子元素4</div>
</div>
</body>
</html>